1. Field of the Invention
This invention relates to an optical transmission system and an optical repeater and more particularly to a long-distance and/or wide-band optical transmission system using optical fibers and an optical repeater used in the optical transmission system.
2. Related Art
Long-distance optical transmission systems such as submarine cables, in general, use a nonreturn-to-zero (NRZ) system that transmits rectangular optical pulses or an optical soliton transmission system that transmits solitary pulses. The optical soliton transmission system transmits ultra-short optical pulses over a long distance while holding their pulse forms unchanged, by balancing the non-linearity characteristics and the wavelength dispersion (or chromatic dispersion) characteristics of optical fibers. There is a report on a successful transmission over the ultra-long distance as long as 14,000 km by ultra-high speed transmission as high as 20 gigabits/second.
In any of the transmission systems, optical fibers for transmission are designed so that their wavelength dispersion characteristics be a limited value for the signal wavelength .lambda.s, in order to prevent deterioration in transmission characteristics caused by the non-linearity of the optical fibers. However, since the wavelength dispersion has a certain value other than zero, the wave dispersion value accumulates and increases gradually according to the transmission distance, and the transmission waveform has been largely deteriorated.
Under the circumstances, there is a proposal that limits the cumulative value of wavelength dispersion within a predetermined value. This proposal uses dispersion compensating fibers (equalizing fibers) inserted among transmission optical fibers and having the opposite wavelength dispersion characteristics, such that wavelength dispersion as a whole becomes zero or quite small. FIG. 10 schematically shows a system of this type. Connected between an optical transmitter terminal 110 and an optical receiver terminal 112 are transmission optical fibers 114 (optical fibers called dispersion-shifted fibers whose wavelength dispersion becomes substantially zero in the band of 1.5 .mu.m) via optical amplifying repeaters 116. An equalizing fiber 118 is connected in each equalizing interval. For example, a single transmission optical fiber is about 40 km long, and each equalizing interval is about 500 km in NRZ transmission system and 200 km in optical soliton transmission system, although the values depend on various system designs.
FIGS. 11 and 12 show a graph of cumulative wavelength dispersion values varying with transmission distance. FIG. 11 shows the case of NRZ system, and FIG. 12 shows the case of optical soliton system, taking cumulative wavelength dispersion on vertical axes and transmission distance on horizontal axes. Typically, optical fibers used in NRZ system are designed such that their wavelength dispersion be accumulated on the minus side while those in optical soliton system are designed such that their wavelength dispersion be accumulated in the plus side.
FIG. 13 schematically shows a construction of the optical amplifier repeater 116 for the band of the wavelength 1,500 nm. An output laser beam from a pumping LD (laser diode) module 124 is introduced to an erbium-doped fiber 120 via a WDM (wavelength division multiplex) coupler 122 to excite the erbium-doped fiber 120. Located downstream of the WDM coupler 122 is an optical isolator 126 for blocking off any reflective light. Some optical soliton transmission systems further include an optical filter positioned downstream of the optical isolator 126. The optical filter is a band pass filter that permits optical signals in the band of the signal wavelength alone to pass through.
A wavelength division multiplexing system is considered useful as means for further increasing the transmission capacity. If the wavelength division multiplexing system is applied to the optical transmission line relying on wavelength dispersion compensation as shown in FIG. 10, then the following disadvantages occur. That is, since both the transmission optical fiber 114 and the equalizing fiber 118 have wavelength dispersion characteristics that exhibit a positive inclination relative to the wavelength, even if wavelength dispersion of a certain wavelength can be compensated to zero by the equalizing fiber 118, wavelength dispersion of other wavelengths cannot be reduced to zero.
FIG. 14 schematically shows wavelength characteristics of wavelength dispersion of the transmission optical fiber 114 and the equalizing fiber 118, taking wavelength dispersion on the vertical axis and wavelengths on the horizontal axis. For NRZ transmission system, the transmission optical fiber 114 is designed such that its cumulative wavelength dispersion in each equalizing interval approximately exhibits the characteristics A while the equalizing fiber 118 is designed to have the characteristics B. For optical soliton transmission system, in contrast, the transmission optical fiber 114 is designed such that its cumulative wavelength dispersion in each equalizing interval exhibits the characteristics B while the equalizing fiber 118 is designed to have the characteristics A. In the design, of course, wavelength dispersion in each unit distance is determined such that the cumulative value in each transmission distance be reduced to zero or quite a small value by addition.
It is known from FIG. 14 that, even if the wavelength dispersion of the transmission optical fiber 114 can be reduced to zero by the equalizing fiber 118 in regard of a certain wavelength .lambda.m, wavelength dispersion in regard of adjacent other wavelengths .lambda.a and .lambda.b cannot be reduced to zero. This difference is accumulated according to the transmission distance, and amounts to a non-negligible value. FIGS. 15 and 16 shows cumulative wavelength dispersion in relation to the transmission distance when three wavelengths, 1,555 nm, 1,558 nm and 1,561 nm are wavelength-division-multiplexed. FIG. 15 shows the case of NRZ system and FIG. 16 shows the case of optical soliton transmission system. Both graphs take cumulative wavelength dispersion on vertical axes and transmission distance on horizontal axes.
Preferable wavelength dispersion in every 1 km in NRZ transmission system is considered -2.0 ps/nm approximately. In a certain design employing the wavelength division multiplexing system, wavelength dispersion per 1 km is -2.0 ps/nm for the wavelength of 1,558 nm, -2.3 ps/nm for the wavelength of 1,555 nm, -1.7 ps/nm for the wavelength of 1,561 nm, and inclination of the wavelength dispersion in 1 km relative to the wavelength is 0.1 ps/nm/nm. That is, the difference of 3 nm in signal wavelength results in the difference of 0.3 ps/nm in wavelength dispersion in every 1 km. If the full length of the transmission optical fiber 114 in each equalizing interval is 500 km, and if the equalizing fiber 118 reduces cumulative wavelength dispersion for the wavelength .lambda.s=1,558 nm into zero, cumulative wavelength dispersion of a shorter wavelength deviates in the minus direction, and that of a longer wavelength deviates in the plus direction as shown in FIG. 15. That is, in regard of wavelengths other than the center wavelength .lambda.s=1,558 nm, even the equalizing fiber 118 cannot fully compensate the cumulative wavelength dispersion, and the offset value of cumulative wavelength dispersion increases with the transmission distance.
Preferable wavelength dispersion in every 1 km for optical soliton transmission system is considered 0.4 ps/nm approximately. In a certain design employing the wavelength division multiplexing system, wavelength dispersion in 1 km is 0.4 ps/nm for the wavelength of 1,558 nm, 0.19 ps/nm for the wavelength of 1,555 nm, 0.61 ps/nm for the wavelength of 1,561 nm, and inclination of the wavelength dispersion in every 1 km relative to the wavelength is 0.07 ps/nm/mn. Then, the difference of 3 nm in signal wavelength results in the difference of 0.21 ps/nm in wavelength dispersion in 1 km. If the full length of the transmission optical fiber 114 in each equalizing interval is 200 km, and if the equalizing fiber 118 reduces cumulative wavelength dispersion for the wavelength .lambda.s=1,558 nm into zero, cumulative wavelength dispersion of a shorter wave length deviates in the minus direction, and that of a longer wavelength deviates in the plus direction as shown in FIG. 16. Even in the optical soliton transmission system, in regard of wavelengths other than the center wavelength .lambda.s=1,558 nm, the equalizing fiber 118 cannot fully compensate the cumulative wavelength dispersion, and the offset value of cumulative wavelength dispersion increases with the transmission distance.